I sell a lot on eBay and if the shipping rate is listed as a specific dollar amount, they need to honor that quote. It is not your fault it costs more to ship. If the amount for shipping was not listed, they can charge you the going rate.
I normally put the weight of the item into ebay's shiping sections and let the shipping calculator do the rest. if people don't want to pay high shipping charges, then by something closer or pick up the item.
If I put a fixed shipping amount in, I am required to honor it. Regardless if it costs me more or less to ship the item
Check with DHL. They seem to be cheaper for shipping large items. but don't pay more than $100.
Otherwise file a complaint with eBay.